The invention concerns a protective layer for load-transferring contact surfaces of gas turbine components made of alloys based on titanium, nickel, cobalt or iron against friction wear, friction corrosion and/or friction fatigue.
The surfaces of component pairs with positive fit in gas turbine power plants, especially in the area of rotating blade foot/disk groove, tend to undergo fretting wear, i.e., friction corrosion or friction wear when oscillating at high frequencies at low amplitudes and at high temperatures. Temperatures above approximately 450xc2x0 C. strongly accelerate wear. In bad cases, cracks caused by fretting fatigue, i.e., friction fatigue, can lead to premature failure of the component.
U.S. Pat. No. 5,312,696 (Beers et al.) discloses a protective layer against fretting wear for components made of nickel, cobalt or titanium alloys, which protective layer consists of copper and aluminum, possibly with silicon as an additional alloy element. The percent by weight of these alloy elements lies within the following ranges:
Preferred compositions of these aluminum bronzes without added silicon contain 92% Cu and 8% Al or 92.5% Cu and 7.5% Al. It was shown that layers According to this U.S. Patent possess sufficient high-temperature oxidation resistance, however their heat resistance drops drastically above 450xc2x0 C. The surface pressure at the blade foot/disc groove eats away such layers from the contact zone and causes them to fail (cake dough effect).
German Patent 42 29 600 C1 (Grunke et al.) concerns a layer structure for titanium components with a metal intermediate layer and outer protective layer. The latter is formed by a copper/aluminum alloy or other alloys, preferably with a composition of CuAl8. Apart from the additionally requited intermediate layer, the same considerations apply to the cover layer of aluminum bronze as discussed above with reference to the U.S. Patent. The other alloys for alternative protective/cover layers are not bronzes, i.e. not copper alloys, and will therefore not be further discussed in this context.
In view of the above prior art protective layers and their disadvantages, the problem of the invention is to present a protective layer based on a copper/aluminum alloy for load-transferring contact surfaces of components, especially titanium components that strongly resist oxidation, corrosion and friction wear, especially at temperatures above 450xc2x0 C., that is therefore particularly useful in gas turbine drives.
This problem is solved by the present protective layer formed by an alloy that contains copper as the main element and aluminum and chromium as additional elements. Iron as an additional element can further improve the protective properties. The element chromium (Cr) with 2-5% parts by weight particularly increases the heat resistance and the high-temperature oxidation resistance while simultaneously leaving the protective layer sufficiently ductile. More specifically, according to the invention there is provided a gas turbine component for operation at temperatures above at least 450xc2x0 C., said gas turbine component comprising a base structure made of alloys made of alloying elements selected from the group consisting of titanium, nickel, cobalt and iron, said base structure comprising a contact surface for cooperation with other turbine components, said gas turbine component further comprising a protecting layer on said contact surface for protection against fretting, wherein said protecting layer is made of an alloy consisting of aluminum 4 to 8% by wt., chromium 2 to 5% by wt., iron 0 to 3.5% by wt., the remainder being copper plus unavoidable impurities.